Fluid purification, separation or reaction processes using active materials such as adsorbents and/or catalysts are well known in the art and there are multiple vessel designs in use today for these types of processes. Examples of such vessels include both vertically and horizontally oriented cylindrical vessels with upward or downward gas flow through the bed of adsorbent and/or catalytic material during the process. A third type of vessel, as employed herein, is oriented with a vertical central or longitudinal axis and an internal design that directs the process gas flow radially through the bed. This radial flow design consists of a cylindrical pressure vessel enclosing gas permeable concentric inner and outer baskets to contain a bed of one or more layers of active material. One common use and example of these vessels is in adsorption processes for the separation or purification of gases.
As the size of these systems, particularly adsorption based gas separation systems, increases to meet the growing product demand; there is a need to provide larger pressure vessels without significantly increasing the footprint (ground area requirements) of the vessels. This is a difficult challenge because the higher fluid throughput demands a proportional increase in the frontal flow area of the vessels. Radial flow designs offer the ability to increase frontal flow area by increasing the height of the vessel without substantially altering the vessel footprint. Furthermore, radial flow designs offer a more efficient means of increasing flow area compared to either horizontal or axial flow vessel designs.
One commercial example is the increasing demand for larger cryogenic air separation units (ASUs) to meet the growing needs for large quantities of oxygen and nitrogen used in various industrial processes. ASUs require front end purification vessels (adsorption vessels) to purify the feed air stream by removing carbon dioxide, water, trace hydrocarbons and other contaminants prior to the air entering the ASU. This removal is typically accomplished through gas adsorption processes. Larger ASUs require larger “prepurification units”, as they are commonly known; to treat the incoming feed air prior to the cryogenic distillation process. Larger units present a challenge to vessel designers when trying to control the size of the vessel because higher throughput of feed air demands a proportional increase in the frontal flow area provided by the vessels resulting in larger, more costly vessels. While many types of vessel designs are used in these prepurification units, the radial flow designs are of the most commercial interest for large ASU applications.
Radial flow vessels, also known as “radial bed vessels”, are characterized by a packed bed of active material contained between at least two concentric cylindrical porous or perforated members fixed within the vessel. These cylindrical members are commonly referred to as “baskets” and contain the active material there between. Fluid such as a gas containing two or more components enters either at the top or bottom of the pressure vessel and is directed into an outer channel formed between the solid wall of the pressure vessel and the cylindrical porous outer basket. The fluid then flows radially through the porous wall of the outer basket, through the packed bed of active material and exits through the porous wall of the inner basket into a central channel (inner channel) aligned with the axis of the pressure vessel. The fluid then exits the pressure vessel at the top or bottom of the pressure vessel as desired. Alternatively, fluid flow can be directed into an inner channel and exits the bed through the outer channel. If such vessel is used in an adsorption based gas purification or separation process, feed and purge (regeneration) gases typically flow in the reversed direction to each other through the bed and channels.
Radial bed vessels can be designed to provide low pressure drop and can accept higher flow rates without the threat of fluidizing the material in the bed, i.e. because the active material is inherently constrained in the direction of flow by the concentric baskets. These materials can be “packed” densely within the bed to create low void volume and to improve process efficiency. However, such vessels and the packed beds within may suffer from non-uniform or inconsistent fluid flow distribution if designed improperly. This flow problem, known as flow maldistribution, leads to early breakthrough of impurities in adsorbers, low conversion efficiency in chemical reactors or generally less-desired fluid passing through the active material bed, i.e. resulting in lower purity product and lower process efficiency with the corresponding inefficient use of the bed.
Numerous methods have been previously employed to improve the flow distribution in the radial bed vessels. For example, U.S. Pat. No. 5,759,242 utilizes a tapered outer vessel wall to create a conical outer channel and hence to achieve enhanced flow distribution. U.S. Pat. Nos. 4,541,851 and 5,827,485 insert a conical distributor element within the inner channel to create similar effect in the inner channel. To further improve the flow distribution in a radial flow vessel with tapered outer channel, U.S. Pat. No. 7,128,775 uses variable perforation patterns on the inner and outer baskets. Alternatively, U.S. Pat. No. 5,814,129 uses an elongated perforated baffle inserted into the channels to enforce a serpentine flow path within the inner or the outer channel. These patents utilize complex mechanical designs and/or additional equipment installed within the vessel in an attempt to improve flow distribution. Such means complicate fabrication and increase cost of the vessel. Moreover, none of these patents disclose design guidelines, criteria, or range of operation conditions necessary to achieve uniform flow distribution within a radial bed vessel. The phrases “radial bed vessels” and “radial bed reactors” are used interchangeably herein to incorporate all of the processes included in the present invention.
Several academic studies have investigated distribution of flow in radial flow reactors. In a series of studies, Heggs et al. (Gas Sep. Purif., vol. 8, no. 4, 257-264 (1994), Gas Sep. Purif., vol. 9, no. 3, 171-180 (1995), Gas Sep. Purif., vol. 9, no. 4, 243-252 (1995)) investigated radial flow distribution in a small (0.34 m diameter, 0.26 m length) annular carbon bed by developing a model of the flow to predict bed and channel pressure profiles. The ratio of the center pipe to outer annulus cross-sectional areas was 0.42 and the maximum pressure drop across the bed was 720 Pa (0.1 psi).
Chang et al. (AIChE J., vol. 29, no. 6, 1039-1041 (1983)) conducted an analytical study of small radial flow fixed-bed reactors (0.12 m diameter, 1.0 m length) to determine the effect of non-uniform flow distribution upon reactor conversion efficiency. They concluded that “π-flow” (flow in inner and outer channels in opposite directions) is always better than “z-flow” (flow in the same direction in both inner and outer channels). It was suggested that an ideal or optimum flow profile would be obtained when the ratio of center pipe and annulus cross sectional areas was equal to one and that the ratio of channel pressure drop to bed pressure drop was equal to zero, i.e. channel resistances equal to zero. However, Chang, et al. also noted that channel resistance is finite and typically of the order of 20% of the bed resistance. The effects of catalyst porosity and vessel diameter upon flow distribution were characterized in terms of the pressure drop ratio and the difference in the flow between the center pipe and outer annulus.
More recently Kareeri et al. (Ind. Eng. Chem. Res., 45, 2862-2874 (2006)) introduced computational fluid dynamics (CFD) to investigate the effect of flow distribution upon the “pinning” phenomena in radial flow moving bed reactors. Kareeri, et al. conducted a survey of the literature (those noted above as well as many others) and concluded that “previous analytical and numerical models for studying the flow distribution in a radial flow reactor are limited and rather simplified.” As a result, 3-D CFD models were developed and used to study the flow distribution in small radial flow reactors (0.5 m diameter, 1.68 m length) with maximum bed pressure drop less than about 160 Pa (0.023 psi).
While the academic studies provide tools and methodologies for predicting pressure and flow profiles, the applications have been predominately aimed at chemical reactors and limited to very small scale vessels. “π-flow” configurations have been consistently recommended. The low flows and small channel resistances resulting from these studies of small reactors inherently produce minimal flow maldistribution. The very low bed resistances are too low to be practical or representative of beds in large industrial scale reactors and purifiers. None of these studies address the issues that arise when the inlet and exit flows are appreciably different, e.g. in bulk separations. Thus, there is a need to quantify vessel and flow parameters required to achieve uniform flow distribution in industrial scale reactors and purifiers applicable to a variety of flow configurations (e.g. “z-flow” and “π-flow”). Furthermore, such methodology must succeed within the imposed structural, induced thermal and manufacturing requirements inherent in industrial scale vessels and processes. The present invention addresses these needs.
It is essential to achieve uniform fluid flow distribution through industrial scale radial bed vessels for successful operation of these processes. The vessel geometry (including the size of inner and outer channels and overall vessel diameter); the bed height and the bed transfer length; the packed bed properties (such as the average particle size and the bed void fraction or bed porosity); gas properties (such as density and viscosity); and the process conditions (such as flow rate, pressure and temperature of the fluid) all contribute to the flow distribution in radial bed vessels. A proper vessel design should result in uniform flow distribution of both the feed and purge (regeneration) fluid flows. Thus, radial bed vessels are provided herein with properly designed inner and outer channels and bed pressure drop requirements leading to vessels that are less expensive and easier to fabricate while achieving better flow distribution.
In addition to affecting flow distribution, the vessel geometry has also an important impact on achieving optimum process performance. For example, cycle time in cyclic separation processes and conversion efficiency in reaction processes depend upon the transfer length or depth of the bed. In other words, flow distribution and process performance are interrelated or coupled and this relationship may vary with the type of process. “z-flow” configuration is often desired as may be dictated by external heat loss considerations, process piping requirements, etc. The prior teachings have struggled with such complexities and thus no clear teaching for uniform flow distribution has been identified. Such deficiencies are overcome in the present invention wherein specific criteria have been developed which may be applied universally in radial bed configurations to achieve relatively uniform flow distribution and therefore overcome any degradation in process performance caused by maldistribution of fluid flows.